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docker-net-dhcp

docker-net-dhcp is a Docker plugin providing a network driver which allocates IP addresses (IPv4 and optionally IPv6) via an existing DHCP server (e.g. your router).

When configured correctly, this allows you to spin up a container (e.g. docker run ... or docker-compose up ...) and access it on your network as if it was any other machine! Probably not a great idea for production, but it's pretty handy for home deployment.

Usage

Installation

The plugin can be installed with the docker plugin install command:

$ docker plugin install ghcr.io/devplayer0/docker-net-dhcp:release-linux-amd64
Plugin "ghcr.io/devplayer0/docker-net-dhcp:release-linux-amd64" is requesting the following privileges:
 - network: [host]
 - host pid namespace: [true]
 - mount: [/var/run/docker.sock]
 - capabilities: [CAP_NET_ADMIN CAP_SYS_ADMIN CAP_SYS_PTRACE]
Do you grant the above permissions? [y/N] y
release-linux-amd64: Pulling from ghcr.io/devplayer0/docker-net-dhcp
Digest: sha256:<some hash>
<some id>: Complete
Installed plugin ghcr.io/devplayer0/docker-net-dhcp:release-linux-amd64
$

Note: If you get an error like invalid rootfs in image configuration, try upgrading your Docker installation.

Other tags

There are a number of supported tags for different architectures and versions, the format is <version>-<os>-<architecture>. For example, latest-linux-arm-v7 would install the newest build for ARMv7 (e.g. for Raspberry Pi).

Version

  • release: The latest release (can be upgraded via docker plugin upgrade)
  • x.y.z: A specific (semver) release (e.g. 0.1.0)
  • latest: Build of the newest commit

OS

Currently only linux is supported.

Architecture

  • amd64: Intel / AMD 64-bit
  • 386: Intel / AMD legacy 32-bit
  • arm64-v8: ARMv8 64-bit
  • arm-v7: ARMv7 (e.g. Raspberry Pi)

Unfortunately Docker plugin images don't support multiple architectures per tag.

Network creation

In order to create a Docker network using net-dhcp, you'll need a pre-configured bridge interface on the host. How you set this up will depend on your system, but the following (manual) instructions should work on most Linux distros:

# Create the bridge
$ sudo ip link add my-bridge type bridge
$ sudo ip link set my-bridge up

# Assuming 'eth0' is connected to your LAN (where the DHCP server is)
$ sudo ip link set eth0 up
# Attach your network card to the bridge
$ sudo ip link set eth0 master my-bridge

# If your firewall's policy for forwarding is to drop packets, you'll need to add an ACCEPT rule
$ sudo iptables -A FORWARD -i my-bridge -j ACCEPT

# Get an IP for the host (will go out to the DHCP server since eth0 is attached to the bridge)
# Replace this step with whatever network configuration you were using for eth0
$ sudo dhcpcd my-bridge

Once the bridge is ready, you can create the network:

$ docker network create -d ghcr.io/devplayer0/docker-net-dhcp:release-linux-amd64 --ipam-driver null -o bridge=my-bridge my-dhcp-net
<some network id>
$

# With IPv6 enabled
# Although `docker network create` has a `--ipv6` flag, it doesn't work with the null IPAM driver
$ docker network create -d ghcr.io/devplayer0/docker-net-dhcp:release-linux-amd64 --ipam-driver null -o bridge=my-bridge -o ipv6=true my-dhcp-net
<some network id>
$

Note: The null IPAM driver must be used, or else Docker will try to allocate IP addresses from its choice of subnet - this can cause IP conflicts since the bridge is connected to your local network!

Container creation

Once you've set up a network, you can create some containers:

$ docker run --rm -ti --network my-dhcp-net alpine
/ # ip address show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
159: my-bridge0@if160: <BROADCAST,MULTICAST,UP,LOWER_UP,M-DOWN> mtu 1500 qdisc noqueue state UP qlen 1000
    link/ether 86:41:68:f8:85:b9 brd ff:ff:ff:ff:ff:ff
    inet 10.255.0.246/24 brd 10.255.0.255 scope global test0
       valid_lft forever preferred_lft forever
/ # ip route show
default via 10.255.0.123 dev my-bridge0
10.255.0.0/24 dev my-bridge0 scope link  src 10.255.0.246
/ #

Or, in a Docker Compose file:

version: '3'
services:
  app:
    hostname: my-http
    image: nginx
    mac_address: 86:41:68:f8:85:b9
    networks:
      - dhcp
networks:
  dhcp:
    external:
      name: my-dhcp-net

The above Compose file assumes your network has already been created with docker network create. This is the recommended way to use docker-net-dhcp, since it allows the network to be shared among multiple compose projects and other containers. However, you can also create the network as part of the Compose definition. In this case Docker Compose will manage the network itself (for example deleting it when docker-compose down is run).

version: '3'
services:
  app:
    image: nginx
    hostname: my-server
    networks:
      - dhcp
networks:
  dhcp:
    driver: ghcr.io/devplayer0/docker-net-dhcp:release-linux-amd64
    driver_opts:
      bridge: my-bridge
      ipv6: 'true'
      ignore_conflicts: 'false'
      skip_routes: 'false'
    ipam:
      driver: 'null'

Note:

  • It will take a bit longer than usual for the container to start, as a DHCP lease needs to be obtained before creating it
  • Once created, a persistent DHCP client will renew the DHCP lease (and then update the default gateway in the container) when necessary - this client runs separately from the container
  • Use --mac-address to specify a MAC address if you've configured reserved IP addresses on your DHCP server, or if you want a container to re-use an old lease
  • Add --hostname my-host to have the DHCP transmit this name as the host for the container. This is useful if your DHCP server is configured to update DNS records from DHCP leases.
  • If the docker run command times out waiting for a lease, you can try increasing the initial timeout value by passing -o lease_timeout=60s when creating the network (e.g. to increase to 60 seconds)
  • By default, a bridge can only be used for a single DHCP network. There is additionally a check to see if a bridge is is used by any other Docker networks. To disable this check (it's also possible this check might mistakenly detect a conflict), pass -o ignore_conflicts=true when creating the network.
  • docker-net-dhcp will try to copy static routes from the host bridge to the container. To disable this behaviour, pass -o skip_routes=true when creating the network.

Debugging

To read the plugin's log, do cat /var/lib/docker/plugins/*/rootfs/var/log/net-dhcp.log (as root). You can also use docker plugin set ghcr.io/devplayer0/docker-net-dhcp:release-linux-amd64 LOG_LEVEL=trace to increase log verbosity.

Implementation

Fundamentally, the same mechanism is used by net-dhcp as Docker's bridge driver to wire up networking to containers. That is, a bridge on the host is used as a switch so that containers can communicate with each other - veth pairs connect each container's network namespace to the bridge.

  • While Docker creates and manages its own bridges (and routes and filters traffic), net-dhcp uses an existing bridge on the host, bridged with the desired local network.
  • Instead of allocating IP addresses from a static pool stored on the Docker host, net-dhcp relies on an external DHCP server to provide IP addresses

Flow

  1. Container creation request is made
  2. A veth pair is created and the host end is connected to the bridge (at this point both interfaces are still in the host namespace)
  3. A DHCP client (BusyBox udhcpc) is started on the container end (still in the host namespace) - initial IP address is provided to Docker by the plugin
  4. Docker moves the container end of the veth pair into the container's network namespace and sets the IP address - at this point udhcpc must be stopped
  5. net-dhcp starts udhcpc on the container end of the veth pair in the container's network namespace (but still in the plugin PID namespace - this means that the container can't see the DHCP client)
  6. udhcpc continues to run, renewing the lease when required, until the container shuts down